Polymethyl methacrylate (PMMA) bone cements are based on the fundamental work done by Sir Charnley (Charnley, J.: Anchorage of the femoral head prosthesis of the shaft of the femur. J. Bone Joint Surg. 42 (1960) 28-30). Conventional polymethyl methacrylate bone cements (PMMA bone cements) are composed of a powdery component and a liquid monomer component (K.-D. Kuhn: Knochenzemente für die Endoprothetik: Ein aktueller Vergleich der physikalischen and chemischen Eigenschaften handelsüblicher PMMA-Zemente. Springer-Verlag Berlin Heidelberg New York, 2001). The monomer component generally contains the monomer methyl methacrylate and an activator dissolved therein (N,N-dimethyl-p-toluidine). The powder component, also called cement powder or bone cement powder, contains one or more polymers which are manufactured by polymerisation, for example suspension polymerisation, on the basis of methyl methacrylate and co-monomers such as styrene, methyl acrylate or similar monomers, an X-ray opaque component and the initiator dibenzoyl peroxide. When the powder component is mixed with the monomer component, the swelling of the polymers of the powder components in the methyl methacrylate creates a plastically workable paste; this paste is the actual bone cement or bone cement paste. When the powder component is mixed with the monomer component, the N,N-dimethyl-p-toluidine activator reacts with dibenzoyl peroxide forming radicals in the process.
The radicals formed initiate the radical polymerisation of the methyl methacrylate. As the polymerisation of the methyl methacrylate progresses, the viscosity of the bone cement paste increases until it sets.
PMMA bone cements can be mixed in suitable mixing vessels with the aid of spatulas by mixing the cement powder with the monomer liquid. This may result in the inclusion of air bubbles in the bone cement paste, which can have a negative impact on the mechanical properties of the bone cement when it has set.
A large number of vacuum cementing systems have been disclosed whose aim was to prevent air inclusions in the bone cement paste; the following systems are stated by way of example: U.S. Pat. Nos. 6,033,105A, 5,624,184A, 4,671,263A, 4,973,168A, 5,100,241A, WO 99/67015 A1, EP 1 020 167 A2, U.S. Pat. No. 5,586,821A, EP 1 016 452 A2, DE 36 40 279 A1, WO 94/26403 A1, EP 1 005 901 A2, EP 1 886 647 A1, U.S. Pat. No. 5,344,232 A.
A further development in the cementing technique are cementing systems where the cement powder as well as the monomer liquid are already packed in separate compartments of the mixing devices and mixed with each other in the cementing system only when the cement is to be applied immediately. Such closed full pre-packed mixing devices were proposed with EP 0 692 229 A1, DE 10 2009 031 178 B3, U.S. Pat. Nos. 5,997,544 A, 6,709,149 B1, WO 00/35506 A1, EP 0 796 653 A2 and U.S. Pat. No. 5,588,745 A.
The patent DE 10 2009 031 178 B3 discloses a storage and mixing device as a full pre-packed mixing device, in which the starting components necessary to produce the bone cement paste are already stored in the storage and mixing device and can be brought together and mixed in the storage and mixing device. The storage and mixing device has a two-part dispensing plunger to close a cement cartridge. A combination of a gas-permeable sterilisation plunger and a gas-impermeable sealing plunger is used here.
Polymethyl methacrylate bone cements are applied in the not yet fully set, pasty state as bone cement paste after the cement powder has been mixed with the liquid monomer components. When mixing devices are used, the bone cement paste is located in a cartridge when the cement used is a powder-liquid cement. When these conventional PMMA bone cements are applied, the bone cement paste formed after the two starting components have been mixed is extruded with the aid of manually operated extrusion devices. The bone cement paste is extruded from the cartridge by moving a dispensing plunger. The dispensing plungers usually have a diameter of 30 mm to 40 mm and therefore an area of 7.0 cm2 to 12.5 cm2 on the outside, where a pushrod or a rod of the extrusion device acts during the extrusion process. The movement of the dispensing plunger is, for example, brought about by manually operated, mechanical extrusion devices. These manual extrusion devices normally achieve an extrusion force in the range of approx. 1.5 kN to 3.5 N.
These simple mechanical extrusion devices use clamping rods for the extrusion, which are driven by a manually operated toggle lever. The manually driven extrusion devices have been tried-and-tested around the globe for many decades and currently represent the Prior Art. The advantage of these extrusion devices is that the medical user has a feeling for the penetration resistance of the bone cement paste into the bone structures (cancellous bone) via the manual force they need to apply.
When using all the full pre-packed mixing devices known to date, the medical user has to carry out several operating steps on the devices in a predetermined order one after the other, until the mixed bone cement paste is available and can be applied. Executing the operating steps in the incorrect order can cause the mixing device to fail and thus cause disruptions to the surgical operating procedure. Expensive training courses for the medical users are therefore required to prevent user errors.
WO 00/35506 A1 proposes a device where the polymethyl methacrylate cement powder is stored in a cartridge, the cement powder filling the whole volume of the cartridge and the spaces between the particles of the cement powder having a volume which corresponds to the volume of the monomer liquid which is necessary to produce bone cement paste with the cement powder stored in the cartridge. This device is designed such that the monomer liquid is introduced into the cartridge from the top through the action of a vacuum, a vacuum being applied for this purpose at a vacuum connector on the underside of the cartridge. The monomer liquid is thereby pulled through the cement powder, whereby the air in the spaces between the cement powder particles is displaced by the monomer liquid. This obviates the need for the cement paste formed to be subjected to a thorough mechanical mixing with a mixer.
The disadvantage of this system is that cement powders which swell quickly with the monomer liquid cannot be mixed with this device, because the fast swelling particles of cement powder form a gelatinous barrier after the monomer liquid has penetrated around 1 to 2 cm into the cement powder and hinder the migration of the monomer liquid through all of the cement powder. Conventional cement powders additionally exhibit the phenomenon that the particles of cement powder are wetted only badly by methyl methacrylate because they have different surface energies. This means the methyl methacrylate only penetrates relatively slowly into the cement powder. When a vacuum is used, it is furthermore not possible to exclude the fact that after the monomer liquid has completely penetrated through the cement powder, the monomer liquid is removed by suction via the vacuum connection. Insufficient monomer liquid is then available for the paste to set by radical polymerisation, or the ratio of the mixture is changed unintentionally and hence the consistency of the bone cement paste, too. A further problem is that the air trapped between the cement powder particles by the monomer liquid should be displaced from the top to the bottom, because the air, which has a lower specific weight than the monomer liquid, has the tendency because of gravity to migrate to the top in the cement powder and not to the bottom towards the vacuum connection.
DE 10 2016 121 607, which was not published in advance, proposes a full pre-packed mixing system with a cartridge containing a cement powder to produce a bone cement paste. A dispensing plunger is provided in the cartridge, and a receptacle with a monomer liquid container is arranged behind the cartridge. On the rear of the receptacle is a pumping plunger which can be used to squash the monomer liquid container and to press the monomer liquid out of the receptacle and into the cartridge.
Practical tests have illustrated that the bone cement paste produced with this device always has a good consistency when a suitable cement powder is used. If the squashed monomer liquid container is compressed to the maximum extent as the monomer is being transferred, then a good cement paste is reproducibly obtained. If the burst monomer liquid container is not fully compressed, residue of monomer liquid can remain between the dispensing plunger and the pumping plunger within the fragments of the burst monomer liquid container, which can escape at the end of the extrusion of the cement paste by a subsequent post-compression of the burst monomer liquid container as a consequence of an axial movement of the pumping plunger towards the dispensing plunger. This monomer liquid residue can change the consistency of the bone cement paste as it is being dispensed. Undesired monomer bubbles can form in the bone cement paste as well.
For these and other reasons, a need exists for the present embodiments.